Crosstalk (or inter-channel interference) is a major source of channel impairment for Multiple Input Multiple Output (MIMO) wired communication systems, such as Digital Subscriber Line (DSL) communication systems.
As the demand for higher data rates increases, DSL systems are evolving toward higher frequency bands, wherein crosstalk between neighboring transmission lines (that is to say transmission lines that are in close vicinity over part or whole of their length, such as twisted copper pairs in a cable binder) is more pronounced (the higher frequency, the more coupling).
Different strategies have been developed to mitigate crosstalk and to maximize effective throughput, reach and line stability. These techniques are gradually evolving from static or dynamic spectral management techniques to multi-user signal coordination (or vectoring).
One technique for reducing inter-channel interference is joint signal precoding: the transmit data symbols are jointly passed through a precoder before being transmitted over the respective communication channels. The precoder is such that the concatenation of the precoder and the communication channels results in little or no inter-channel interference at the receivers.
A further technique for reducing inter-channel interference is joint signal post-processing: the received data symbols are jointly passed through a postcoder before being detected. The postcoder is such that the concatenation of the communication channels and the postcoder results in little or no inter-channel interference at the receivers.
The choice of the vectoring group, that is to say the set of communication lines, the signals of which are jointly processed, is rather critical for achieving good crosstalk mitigation performances. Within a vectoring group, each communication line is considered as a disturber line inducing crosstalk into the other communication lines of the group, and the same communication line is considered as a victim line receiving crosstalk from the other communication lines of the group. Crosstalk from lines that do not belong to the vectoring group is treated as alien noise and is not canceled.
Ideally, the vectoring group should match the whole set of communication lines that physically and noticeably interact with each other. Yet, local loop unbundling (imposed by national regulation policies) and/or limited vectoring capabilities may prevent such an exhaustive approach, in which case the vectoring group would include a sub-set only of all the physically interacting lines, thereby yielding limited vectoring gains.
Signal vectoring is typically performed within an access node, wherein all the data symbols concurrently transmitted over, or received from, all the communication lines of the vectoring group are available. For instance, signal vectoring is advantageously performed within a Digital Subscriber Line Access Multiplexer (DSLAM) deployed at a Central office (CO) or as a fiber-fed remote unit closer to subscriber premises (street cabinet, pole cabinet, building cabinet, etc). Signal precoding is particularly appropriate for downstream communication (toward customer premises), while signal post-processing is particularly appropriate for upstream communication (from customer premises).
Linear signal precoding and post-processing are advantageously implemented by means of matrix products.
For instance, a linear precoder performs a matrix-product of a vector of transmit frequency samples with a precoding matrix, the precoding matrix being such that the overall channel matrix is diagonalized, meaning the off-diagonal coefficients of the overall channel, and thus the inter-channel interference, mostly reduce to zero. Practically, and as a first-order approximation, the precoder superimposes anti-phase crosstalk pre-compensation signals over the victim line along with the direct signal that destructively interfere at the receiver with the actual crosstalk signals from the respective disturber lines.
Similarly, a linear postcoder performs a matrix-product of a vector of received frequency samples with a crosstalk cancellation matrix, the crosstalk cancellation matrix being such that the overall channel matrix is diagonalized too.
It is of utmost importance thus to get an accurate and up-to-date estimate of the respective crosstalk couplings in order to properly mitigate the actual crosstalk.
In the recommendation entitled “Self-FEXT Cancellation (Vectoring) For Use with VDSL2 Transceivers”, ref. G.993.5, and adopted by the International Telecommunication Union (ITU) on April 2010, the transceivers are configured to send downstream and upstream pilot sequences over the so-called SYNC symbols, which occur periodically after every 256 DATA symbols. On a given victim line, error samples, which comprise both the real and imaginary part of the slicer error (or receive error vector) as measured for a specific SYNC symbol on a per tone or group-of-tones basis, are reported to a vectoring controller for further crosstalk estimation. The error samples are correlated with a given pilot sequence transmitted over a given disturber line in order to obtain the crosstalk coefficient from that disturber line. To reject the crosstalk contribution from the other disturber lines, the pilot sequences are made orthogonal to each other, for instance by using Walsh-Hadamard sequences comprising ‘+1’ and ‘−1’ anti-phase symbols. The crosstalk estimates are used for initializing or updating the coefficients of the precoding matrix or of the crosstalk cancellation matrix.
Orthogonal pilot sequences as per G.993.5 recommendation are very effective and always produce accurate and unbiased estimates of the crosstalk channels (initialization) or of the residual crosstalk channels (tracking). Yet, with the use of even broader transmit spectrum for next generation copper-access, second-order effects starts arising.
For instance, when a line of a vectoring group shuts down and corresponding transceivers disconnect from the transmission medium, the impedance change over the discontinued line induces a notable change in the crosstalk channels between the other still-active lines of the vectoring group. Indeed, with the increased carrier frequency, second-order crosstalk, i.e. crosstalk from a disturber line into the discontinued line and then back into another victim line, is no longer negligible. As the impedance changes on the discontinued line from a nominal low-value (typically one hundreds ohms) to a high value (typically a few thousands ohms up to the open circuit impedance), the crosstalk couplings into and from the discontinued line are altered, thereby yielding some residual crosstalk on the other still-active lines of the vectoring group. Thus, a new crosstalk acquisition round, which lasts several super frames, needs to take place after one or more lines are discontinued in order to characterize the new residual crosstalk channels. Meanwhile, communication over the other still-active lines is severely impaired by this residual crosstalk, which may affect the user experience and even lead to line retrains.